The invention concerns a method for producing a superconductive wire out of an initial element, the superconductive wire comprising finally one or more superconductive filaments having a diameter between 2 μm and 5 mm and being enclosed in a metallic matrix, and further comprising at least one highly conductive ohmic element, whereby an elongated intermediate element with polygonal cross section is formed out of the initial element by applying a sequence of swaging, drawing or rolling deformations in a deformation step, and whereby the superconductive filaments are formed by a final reaction heat treatment between 300° C. and 1200° C.
A method for producing a superconductive wire is known from the article by P. Ková{hacek over (c)} et al. “Properties of stabilized MgB2 composite wire with Ti barrier”, Supercond. Sci. Technol. 20(2007)771-776.
The fabrication of MgB2 wires and tapes has reached the industrial level, and kilometer lengths can be produced by means of two competing techniques, the “in situ” and the “ex situ” technique. A recent description of these techniques has been given by E. Collings et al. (Supercond. Sci. Technol. 21(2008)103001).
The “in situ” technique is characterized by the fact that MgB2 phase in the filaments is formed by a reaction between B and Mg powder particles at temperatures between 500 and 1000° C., at the end of the deformation to a wire by conventional swaging, drawing or rolling procedures.
This is in contrast to the “ex situ” technique, where the wire deformation is performed on filaments containing already reacted MgB2 powder particles. The scope of the final heat treatment of “ex situ” wires is thus a sintering of the powder mixture in the filaments for improving the connectivity between the MgB2 grains.
The structure of MgB2 is hexagonal, with a strong anisotropy of the superconducting critical fields Bc2 and Birr. It follows that the superconducting critical current density Jc in a MgB2 wire also shows anisotropic behavior depending on the orientation of the applied magnetic field with respect to the tape surface. In the case of ex situ MgB2 tapes a strong anisotropy of Jc is observed (Lezza et al., IEEE Trans. Appl. Supercond. 15(2005)3196), which is due to the fact the original powder mixture already contains MgB2 crystallites which will align along the wire axis during deformation.
The situation is more complex for in situ tapes, where the texture of the MgB2 phase in the filaments after reaction is influenced by the energy transmitted during the initial homogenization ball milling of the powder mixture the Mg+B+additives. Indeed, high energy ball milling, performed with W balls and vails for times up to 100 hours, transforms up to 30% of the total powder to MgB2 (mechanical alloying). Tapes produced with these powders exhibit a marked anisotropy of Jc, as reported by Ková{hacek over (c)} et al. (Supercond. Sci. Technol. 21(2008)015004).
In powders homogenized by low energy ball milling using agate balls and vails during times up to 4 hours, no MgB2 is observed by X ray analysis. This means that either only a very small amount of MgB2 phase has been formed (<2%) or that the MgB2 grains have a size below 15 nanometer and cannot be detected. The consequence is a very low degree of texturing in the reacted tape, and a low anisotropy of Jc.
The enhancement of the critical current density, Jc, in MgB2 wires has been the subject of numerous publications. The most effective way to enhance Jc in MgB2 wires is the introduction of additives to the initial powder mixtures. Several kinds of additives have been proposed, consisting partly or entirely by Carbon, which substitutes Boron in the MgB2 phase up to 20 at. %. The most known additive is SiC (S. X. Dou et al., Appl. Phys. Lett. 81 (2002) 3419) or Carbon (R. H. T. Wilke et al., Phys. Rev. Lett. 92(2004) 217003).
Other ternary additives have led to an increase of Jc in wires and tapes prepared by the in situ technique: B4C (P. Lezza et al. Supercond. Sci. Technol. 19(2006)1030) and a series of carbohydrates, e.g. malic acid, C4H6O5 (M. S. A. Hossain et al., Supercond. Sci. Technol. 20(2007)L51). Finally, combinations between additives have been introduced, e.g. B4C+SiC (R. Flükiger et al., IEEE Trans. Appl. Supercond. 17(2007)2846) or carbohydrates +SiC (H. Yamada et al., Supercond. Sci. Technol. 20(2007)L30).
For all the above mentioned additives, the partial substitution of Boron by Carbon causes a decrease of the atomic order parameter in the hexagonal MgB2 structure, thus causing an increase of the electrical resistivity and thus a higher critical field. At the same time, a decrease of Tc is observed in the filaments, from ˜39 K for binary MgB2 to ˜28K K for 12 at. % substituted Carbon. It follows that the optimum Jc will be obtained by a compromise between the C content and the value of Tc.
In all prior art published at the present day, deformation and final heat treatment of industrial superconducting MgB2 wires are performed at ambient pressure. In two cases, reaction heat treatments under high pressures have been reported, tapes or pressed pellets having been submitted to a HIP (Hot Isostatic Pressing) treatment (Serquis et al., Appl. Phys. Letters, 82(2003,2847), while bulk samples have been prepared by a multianvil press (Prikhna et al., Physica C: Superconductivity, 372-376(2002)1543). However, this is fundamentally different from the present application, the use of these devices being limited to very small sample dimensions: from 1 to 2 cm3 to <100 mm3, respectively. This excludes their use on long wires.
It has to be noted that both, HIP and hot multianvil processing do not describe a cold compression step on the elongated deformed green sample prior to the final heat treatment.
In contrast to the previous works, it is an object of the present invention to provide a method producing a superconductive wire which leads to a substantial increase of the critical current density Jc, its values being enhanced by a factor up to 2.8 at 4.2K and a magnetic field of 10 T. At the same time, the anisotropy factor Γ=Jc(parallel): Jc(perpendicular), where the magnetic field is applied either parallel or perpendicular to the tape surface, should be almost not affected by this new procedure, which is important in view of industrial applications requiring isotropic or almost isotropic wires or tapes.